Reduce number of calculations when finding a nodes relationships

[lewisbirks]

Whilst investigating performance issues of #862 it was noticed that node.getEnclosingBlocks() was being called twice in NodeRelationshipFinder#getFirstEnclosingNode. Whilst some of the computation of this result was cached, not all of it was which means there is extra work being done that isn't required.

I also noticed that some maps were being passed around that might have once been used as a lookup table but now were only being used to access values.

Some other small cleanup has been done where newer java functionality is available or the code could be simplified.

Testing done

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[timja]

Thanks all LGTM